Hello All!
There are tons of questions and answers about the ComWinTop/JXCT type RS485 Soil sensors here. But they all seem to be missing the part about the configuration of the sensors.
I have had NO trouble at all communicating with these things, getting/setting data, BUT
I can't seem to figure out how to configure the NPK Factor and Offset values.
I can write the values just fine, and they DO change the output. But the datasheet is very vague about what numbers to put where. Here is an example:
My real NPK values should be as follows:
N: 306 mg/kg
P: 1130 mg/kg
K: 1637 mg/kg
The docs show this:
Factor and offset like the formula
Y=AX+B
Y is reading value
X is original value
A is factor
B is offset
I contacted support, they said this:
for example, if original value is 100,
set factor=0.1, offset=5
so the reading = 100*0.1+5=15
I still don't quite get it. If someone has any experiance with this, I would LOVE to learn!!!
(For anyone that wants to say this is not a lab test, I know. It is a super ball-park general guesstimate! But I am looking for numbers that aren't what I am currently getting: 13, 0, 24)
What they seem to be telling you is that you set A (the NPK Factor ) and B (the Offset) when you write those values and if X is what the device reads, then what will be returned is Y calculated with the linear function A X + B
to find A and B you need to make two readings for which you know the output and get the original value
say you expect Y1 when the sensors read X1 and Y2 when the sensor reads X2
then you find A and B by resolving the 2 equation system
Do you know that these are real values, or do you want to read these values?
If your current values are
13
0
24
and you want to get
306
1130
1637
you have to set a factor of
306/13
1130/0 <--- this will fail of course
1637/24
You'll have to determine the offsets as well. So you have to get readings for (at least) 2 known values and calculate factor A and offset B as @J-M-L already pointed out. That's called calibration.
So you have to get a second set of tables for different known and measured NPK values. Did you already learn at school to calculate A and B from two equations:
Okay Okay, I think I understand now!
So in my case:
(real N) 306 is the factor
(currently reads) 26
Great question! Currently If I reset the sensor to factory default, I get these readings:
N: 0,
P: 35,
K: 27
I mentioned that I had updated the factor and offset before, after doing the update the sensor reads:
N: 344
P: 1190
K: 1595
These values are well within reason for me, but the fact of the matter is I have NO CLUE if I did this right or not
I am worried I am a bit dense to figure this out
I don't know these sensors; do you have a linear equation for each N, P and K (ie 6 parameters you can provide to the sensors to tune the output?)
as explained, to calibrate those correctly you need 2 known points from which to infer A and B.
process would be set A to 1 and B to 0.
The sensor will calculate Y = 1.X + 0 ➜ the output is the raw value.
you make the measure for two known output points and because the sensor gave you the X for this know Y you can solve the system of equations and find A and B (which assumes the sensor is linear)
Can you provide a link to the datasheet of your specific sensor?
I just did a quick search on that name, and found a website that's just full of BS. Stick a few metal prongs in the ground and it tells you everything: pH, temp, NPK, etc. If so, I suspect that a dowsing rod is working at least as well.
Specifically, I am able to write values using function code 6, from register 04E8 (start of N factor high byte register) TO 04FE (Potassium(K) offset register) incase anyone in the future needs this info
Here is one of my lab test results (sorry about the language):
Sorry to see yet another of those scams come by. A few crudely sharpened metal prongs of course can not do much of what they claim it does... they even suggest you to write the NPK values to the sensor so you can later read them back!
It is even quite clearly written in the Chinglish datasheet that the NPK values they pretend to measure are utter rubbish:
Also they claim to measure pH... without a pH probe present.
I agree that these sensors are somewhat snake oil. I have been told that all
calculations are simply some sort of math done from the EC reading (not sure what though)
The temp, and moisture levels seem good, but that isn't tough at all to get.
Do you know if there are any actually somewhat reliable sensors for long term use?
I know the ONLY real way to get actual values is a lab test, but I would at very least like to have a guidestone throughout the grow season without having to do manual tests monthly.
Do you think I may have better luck with the dedicated NPK sensor?
I have tried the MuRata Temp/Moisture/EC sensor with great success, but at a $1000USD price point, it is nor reasonablely priced also considering the very limited sensor lifespand.
Thanks for the help!
(also have you used these before???)
To be fair you should assume that the "chinglish" is the issue here and that what they mean is that they provide a way to create a linear adaptation of the output value (the A and B discussed before). They certainly don’t mean that you write the output values.
The prongs act as electrodes, and the electrical conductivity of the soil is influenced by the combined presence of ions, including those from N, P, and K compounds amongst other factors (overall soil composition, and environmental conditions,…).
So given stable conditions, those devices could provide readings in an arbitrary unit on an arbitrary scale rather than providing exact concentrations in ppm and you should be aware that these readings are relative and may need to be correlated with actual nutrient concentrations through calibration.
A linear mapping is a step towards providing something that makes more sense to the user but it’s probably snake oil to think that it’s just a linear map.
If they are using electrodes then it’s probably the same approach.
I don’t know the inner working of these sensors - may be they use different frequencies or amplitudes or differential analysis (three prongs) and have studied how the response varies based on the presence of such or such nutrients and tuned the output or maybe it’s a total scam…
May be you should ask the vendor for scientific papers on their studies ?
It's not just that. The other strong indication is that the N, P and K registers of the sensor are explicitly listed as read/write, while all others are read only. So the sensor is certainly designed to have NPK values written to.
so yes, they seem to say that if you don't trust the N,P,K values but you need them for a monitoring system, you could fake those data right into the sensor...
weird (because I suppose the sensor will overwrite those at the next read request)
@J-M-L
I have attempted to use that to write the data in. I assumed this was an error in the datasheet as
attempting to write data like this seems to have NO affect at all. I have no clue what this does.
N, P and K values are not easy to obtain by a probe.
The K may be possible, that's all in the form of K(+) ions. May work with a glass probe similar to a pH probe (this also tells you why this sensor can not measure pH), which is generally potassium based to begin with.
P is mostly PO4(2-) ions. I really miss superscript/subscript now. Maybe another ion specific glass probe?
N is the hardest. NO3(-), NH4(+) and other ions and compounds are possible here. You have to check for them all.
So for any reasonable measurement you indeed are basically limited to taking a sample and sending it off to the lab.
Luckily however there are much cheaper ways to tell whether you need nutrients. Your plants are excellent sensors. Look at how they're doing, and based on the growth and leaf colour and other indicators you can tell what they need.
